According to Merriam Webster, in the 1860s a 'dead line' was a line within or around a prison. Prisoners would be shot for crossing the 'dead line'.
We can all relate to the anxiety and panic associated with missing a deadline. Our world revolves around the calendar and the clock. You must do A task before X date or it’s curtains for you.
Well, at least for most of us, missing a deadline won't actually kill you. So we got that going for us. Which is nice.
Oh, and happy belated Groundhog Day.